1. Field of the Invention
This invention relates to improved aromatics processing. More particularly, this invention is concerned with regenerating aromatics processing catalysts in the presence of steam in such a manner as to enhance their catalytic activities.
2. Description of the Prior Art
U.S. Pat. Nos. 3,126,422; 3,413,374; 3,598,878; 3,598,879 and 3,607,961 show vapor-phase disproportionation of toluene over various catalysts.
The disproportionation of aromatic hydrocarbons in the presence of zeolite catalysts has been described by Grandio et al. in the Oil and Gas Journal, Vol. 69, Number 48(1971).
The use of a catalyst comprising a crystalline zeolite characterized by a silica to alumina mole ratio of at least about 12 and a constraint index within the approximate range of 1 to 12 for the disproportionation of toluene is described in many patents, such as U.S. Pat. Nos. 4,011,276, 4,016,219, 4,052,476, 4,097,543, and 4,098,837, just to name a few.
Since the announcement of the first commercial installation of Octafining in Japan in June, 1958, this process has been widely installed for the supply of p-xylene. See "Advances in Petroleum Chemistry and Refining", volume 4 page 433 (Interscience Publishers, New York 10061). Demand for p-xylene has increased at remarkable rates, particularly because of the demand for terephthalic acid to be used in the manufacture of polyesters.
Typically, p-xylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Density Freezing Boiling Lbs./U.S. Point .degree.F. Point .degree.F. Gal. ______________________________________ Ethyl benzene -139.0 277.0 7.26 P--xylene 55.9 281.0 7.21 M--xylene -54.2 282.4 7.23 O--xylene -13.3 292.0 7.37 ______________________________________
Principal sources of C.sub.8 aromatics are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but are usually in the range 10 to 32 wt % ethyl benzene with the balance, xylenes, being divided approximately 50 wt % meta, and 25 wt % each of para and ortho.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethyl benzene may be separated by fractional distillation, although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Para xylene is separated from the mixed isomers by fractional crystallization.
As commercial use of para and ortho xylene has increased there has been interest in isomerizing the other C.sub.8 aromatics toward an equilibrium mix and thus increasing yields of the desired xylenes.
The Octafining process operates in conjunction with the product xylene or xylenes separation processes. A virgin C.sub.8 aromatics mixture is fed to a processing combination in which the residual isomers emerging from the product separation steps are then charged to the isomerizer unit and the effluent isomerizate C.sub.8 aromatics are recycled to the product separation steps. The composition of isomerizer feed is then a function of the virgin C.sub.8 aromatic feed, the product separation unit performance, and the isomerizer performance.
The isomerizer unit itself is most simply described as a single reactor catalytic reformer. As in reforming, the catalyst contains a small amount of platinum and the reaction is carried out in a hydrogen atmosphere.
Under recommended design conditions, a considerable volume of hydrogen is introduced with the C.sub.8 aromatics. In order to increase throughput, there is great incentive to reduce hydrogen circulation with consequent increase in aging rate of the catalyst. Aging of catalyst occurs through deposition of carbonaceous materials on the catalyst, with need to regenerate by burning off the coke when the activity of the catalyst has decreased to an undesirable level. Typically the recommended design operation will be started up at about 850.degree. F. with reaction temperature being increased as needed to maintain the desired level of isomerization until reaction temperature reaches about 900.degree. F. At that point the isomerizer is taken off stream and regenerated by burning of the coke deposit.
During regeneration, burning proceeds very slowly with diluted oxidizer medium in order to minimize damage to the catalyst. The several days required for regeneration are non-productive and the catalyst after regeneration is at a reduced activity level. For example, an operation at a hydrogen to hydrocarbon recycle ratio of 6.5 results in a cycle life of about 3 months between regenerations with replacement of the catalyst required after about 1 year, four cycles.
In a typical plant for utilization of Octafining, a mixture of C.sub.8 aromatics is introduced to an ethyl benzene tower wherein the stream is stripped of a portion of its ethyl benzene content, to an extent consistent with retaining all the xylenes in the feed stream without unduly expensive "superfractionation". Ethyl benzene is taken overhead while a bottom stream, consisting principally of xylenes, together with a significant amount of ethyl benzene, passes to a xylene splitter column. The bottoms from the xylene splitter constituted by o-xylene and C.sub.8 aromatics passes to the o-xylene tower from which o-xylene is taken overhead and heavy ends are removed. The overhead from the xylene splitter column is transferred to conventional crystallization separation. The crystallizer operates in the manner described in Machell et al., U.S. Pat. No. 3,662,013 dated May 9, 1972.
Because its melting point is much higher than that of the other C.sub.8 aromatics, p-xylene is readily separated in the crystallizer after refrigeration of the stream and a xylene mixture lean in p-xylene is transferred to an isomerization unit. The isomerization charge passes through a heater, is admixed with hydrogen and the mixture is introduced to the isomerizer.
Isomerized product from the isomerizer is cooled and passed to a high pressure separator from which separated hydrogen can be recycled in the process. The liquid product of the isomerization passes to a stripper from which light ends are passed overhead. The remaining liquid product constituted by C.sub.8 + hydrocarbons is recycled in the system to the inlet of the xylene splitter.
It will be seen that the system is adapted to produce maximum quantities of p-xylene from a mixed C.sub.8 aromatic feed containing all of the xylene isomers plus ethyl benzene. The key to efficient operation for that purpose is in the isomerizer which takes crystallizer effluent lean in p-xylene and converts the other xylene isomers in part to p-xylene for further recovery at the crystallizer.
The Octafining process has been extensively discussed in the literature, for example:
1. Pitts, O. M., Connor, J. E. Leun, L. M., Ind. Eng. Chem., 47, 770 (1955).
2. Fowle, M. J., Bent, R. D., Milner, B. E., presented at the Fourth World Petroleum Congress, Rome, Italy, June 1955.
3. Ciapetta, F. G., U.S. Pat. No. 2,550,531 (1951).
4. Ciapetta, F. G., and Buck, W. H., U.S. Pat. No. 2,589,189.
5. Octafining Process, Process Issue, Petroleum Refinery, 1st Vol. 38 (1959), No. 11, Nov., p. 278.
A typical charge to the isomerizing reactor (effluent of the crystallizer) may contain 17 wt. % ethyl benzene, 65 wt. % m-xylene, 11 wt % p-xylene and 7 wt. % o-xylene. The thermodynamic equilibrium varies slightly with temperature. The objective in the isomerization reactor is to bring the charge as near to theoretical equilibrium concentrations as may be feasibly consistent with reaction times which do not give extensive cracking and disproportionation.
Ethyl benzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. Competing reactions are disproportionation of ethyl benzene to benzene and diethyl benzene, hydrocracking of ethyl benzene to ethane and benzene and hydrocracking of the alkyl cyclohexanes.
The rate of ethyl benzene approach to equilibrium concentration in a C.sub.8 aromatic mixture is related to effective contact time. Hydrogen partial pressure has a very significant effect on ethyl benzene approach to equilibrium. Temperature change within the range of Octafining conditions (830.degree. to 900.degree. F.) has a very small effect on ethyl benzene approach to equilibrium.
Concurrent loss of ethyl benzene to other molecular weight products relates to % approach to equilibrium. Products formed from ethyl benzene include C.sub.6 + naphthenes, benzene from cracking, benzene and C.sub.10 aromatics from disproportionation, and total loss to other than C.sub.8 molecular weight. C.sub.5 and lighter hydrocarbon by-products are also formed.
The three xylenes isomerize much more selectively than does ethyl benzene, but they do exhibit different rates of isomerization and hence, with different feed composition situations the rates of approach to equilibrium vary considerably.
Loss of xylenes to other molecular weight products varies with contact time. By-products include naphthenes, benzene, toluene, C.sub.9 + aromatics and C.sub.5 and lighter hydrocracking products.
Ethyl benzene has been found responsible for a relatively rapid decline in catalyst activity and this effect is proportional to its concentration in a C.sub.8 aromatic feed mixture. It has been possible then to relate catalyst stability (or loss in activity) to feed composition (ethyl benzene content and hydrogen recycle ratio) so that for any C.sub.8 aromatic feed, desired xylene products can be made with a selected suitably long catalyst use cycle.
Another xylene isomerization which has achieved widespread commercial use is low pressure operation in vapor phase. Temperatures employed are in the same range as for Octafining, in the neighborhood of 850.degree. F. Pressures are only that required to equal pressure drop through the downstream recovery towers, heat exchangers and the like. For all practical purposes, this is an atmospheric pressure reaction with reactor inlet pressure of about 30 pounds per square inch, gauge. The catalyst is essentially silica-alumina, the acid amorphous heterogeneous catalyst employed in a number of such acid catalyzed processes. Several advantages for that type of isomerization will be immediately apparent.
The unit cost of catalyst is drastically reduced by omission of platinum. At these low pressures, the reactor vessels are made of inexpensive steel and need no structural provision for resisting pressure stress. The process is practical without introduction of molecular hydrogen and needs no auxiliaries for manufacture and recycle of that gas. These features greatly reduce capital and operating costs and have made the low pressure process essentially competitive with Octafining despite the requirement for large vessels at low pressure and low space velocity and the operating disadvantages inherent in the process.
A primary drawback of low pressure vapor phase isomerization as practiced heretofore is its low tolerance for ethyl benzene in the charge. The catalyst will convert ethyl benzene only at high severities such that unacceptable loss of xylene occurs by disproportionation.
Low pressure isomerization as practiced heretofore accepts a further disadvantage in that the catalyst rapidly declines in activity due to deposition of "coke", a carbonaceous layer masking the active sites of the porous silica-alumina catalyst presently conventional in this operation. The coke can be removed by burning with air to regenerate the activity of the catalyst. Continuity of operation is achieved by the well-known "swing reactor" technique. In this technique, two or more reactors are employed, one of which is on stream, while burning regeneration is conducted on a reactor containing spent catalyst which has lost activity by coke deposition. Cycles of two to four days are common practice using one reactor on stream for that period and then shifting to a freshly regenerated vessel.
Present commercial practice involves many large plants of both the Octafining and low pressure types in a loop of p-xylene separation and recycle of other isomers, together with such quantity of ethyl benzene as may be present, through isomerization and back to p-xylene recovery. The commercial options presently in use are Octafining at high pressure with large quantities of hydrogen or low pressure (essentially atmospheric) isomerization with complicated cycling of a swing reactor and necessity for expensive distillation to remove ethyl benzene from the charge to some acceptable level, usually about 5%.
A further alternative heretofore described is isomerization in liquid phase at a pressure adequate to maintain that phase. Highly active zeolite catalysts are effective under these conditions and demonstrate long cycle life, possibly because precursors of coke are dissolved by the reactant liquid and flushed from the reactor before deterioration to coke. See, for example, Wise, U.S. Pat. No. 3,377,400; Bowes et al., U.S. Pat. No. 3,578,723; and Haag et al., U.S. Pat. No. 3,856,871.
It is further known that zeolites characterized by having a silica to alumina mole ratios of at least 12 and constraint indices within the approximate range of 1 to 12 are very effective as catalysts for the isomerization of xylenes. See Burress, U.S. Pat. No. 3,856,873, Morrison, U.S. Pat. No. 3,856,872; Haag et al., supra; Hayward, U.S. Pat. No. 3,856 874; Mitchell et al., U.S. Pat. No. 4,101,596; Olson et al., U.S. Pat. No. 4,159,282; Nicoletti et al., U.S. Pat. No. 4,159,283; and Tabak et al., U.S. Pat. No. 4,163,028. In the absence of hydrogen, these zeolites accumulate coke on stream in the manner to be expected from knowledge in the art to require short cycle times, when operating outside the bounds of limits now found essential to prolonged on-stream periods.
It is known in the art that the use of steam (water) and/or ammonia can be utilized to modify the activity of acid catalysts, e.g. clays, silica-aluminas and zeolites. Much of the emphasis in the field of catalyst activity modification has been directed towards reducing the activity of catalysts. For example, U.S. Pat. No. 4,016,218 teaches the use reduction of catalytic activity of a class of zeolites having a silica to alumina mole ratio of at least 12 and a constraint index within the approximate range of 1 to 12 by the use of prior thermal treatment. Such prior thermal treatment includes the use of a steam atmosphere.
Hydrogen zeolites of the 1 to 12 constraint index type are generally prepared from their alkyl ammonium and ammonium form precursors by calcining in an inert atmosphere, usually in nitrogen at about 1000.degree. F. The more costly nitrogen atmosphere is chosen over the cheaper heating in air to avoid temperature runaway and steam formation that is known to damage the catalyst and results in lower activity. Small samples in the laboratory can be calcined in air without significant steam damage if the temperature is controlled by a slow heat up and by allowing any steam formed to diffuse away. With this careful first calcination, hydrogen zeolites result that are free of residual nitrogen compounds and have the maximum number of acidic hydroxyl group, which is equal to the number of framework aluminums. Samples thusly prepared are designated "fresh samples". The corresponding catalytic activity of these fresh samples is called "initial activity" and when measured by the alpha (.alpha.) test as described hereinafter, assigned the designation of ".alpha.."
It has long been known that the catalytic activity of hydrogen zeolites can be reduced by high temperature heating and especially by steaming.
It is also known that the deactivation due to steam is more pronounced at higher temperatures and longer reaction times. It is also more pronounced at higher steam pressures. Deactivation in the absence of steam, i.e., in an inert atmosphere, requires more severe conditions than steam deactivation.
Recently it has been found that the use of water can be employed to improve certain zeolite catalyst characteristics, while maintaining catalyst activity levels. U.S. Pat. Nos. 4,149,960 and 4,150,062 describe the use of about 0.5 to about 15 moles of water per mole of feedstock in order to substantially reduce the coking and aging rates of the zeolite catalysts used in the processes of these disclosures.
U.S. Pat. Nos. 3,493,519 teaches a method of using steam for the stabilization of Y-faujasite zeolite. There, a chelating agent was used after steaming to take out the excess aluminum from the zeolite. The resultant catalyst of this process is a hydrothermally stable zeolite catalyst having high hydrocarbon conversion activity.
In U.S. Pat. Nos. 3,546,100, it is disclosed that a rare earth exchanged zeolite hydrocracking catalyst such as zeolites X or Y can be improved with respect to its cracking activity and selectivity by using water in controlled amounts to activate catalyst cracking sites. This disclosure states that the amount of water be maintained during the process such that the water vapor partial pressure is kept at a level of about 10 to about 130 mm. water vapor.